Broadband light sources with high output power are used in many optical communication applications that require a high output power over a wide wavelength spectrum. In wavelength division multiplexing (WDM), a broadband light source is cascaded with a comb filter and a modulator to provide various wavelengths of light signals to code information. In yet another example, broadband light sources are used in the testing and measurement of photonics components and devices.
There are two categories of broadband light sources, with both categories having a basic structure comprising a light emitting source, a spectrum equalizer and an isolator.
In the first category, illustrated in FIG. 1, the broadband light source is created by using Amplified Spontaneous Emission (ASE) brought about through the use of Erbium Doped Fiber Amplifiers (EDFA) 23. The EDFA 23 can be pumped in a forward manner 28, in a backward manner 22, or both the forward and backward manner 28 and 22. Examples of prior art which use broadband light sources of this category include U.S. Pat. No. 6,172,995 issued to Yang on 9 Jan. 2001 and U.S. Pat. No. 5,142,660 issued to Chang et al on 25 Aug. 1992. However, broadband light sources of the first category seldom deliver an output power bigger than 10 dBm over a 70 nm 3-dB bandwidth.
In the second category, illustrated in FIG. 2, light from a Light Emitting Diode (LED) 11 which is driven by a diode driver 10 is coupled into a fiber 13 and outputs through fiber 14 after an isolator 15. A Super Luminance Emitting Diode (SLED) or an Edge Luminance Emitting Diode (ELED) can be used instead of a normal LED 11. For instance, U.S. Pat. No. 6,570,659 issued to Schmitt on 27 May 2003 uses multiple SLEDs 30a, 30b, . . . 30n to form a broadband source as shown in FIG. 3. The light from the plurality of SLEDs is coupled into a polarization combiner 33, where the light is attenuated with different ratios, before it is combined for output 34. By choosing appropriate attenuation ratios, the final output light 34 of the broadband light source gains a wide spectrum. Broadband light sources of the second category are able to provide several tens of mW of output power over a 100 nm 3 dB bandwidth.
However, the light spectrums emitted from the broadband light sources of the above two categories are usually not flat. For high precision optical signal processing, the output spectrum of a broadband light source should ideally be as flat as possible over a wide range of wavelengths. As such, there is a need to equalise the output spectrums of the broadband light sources of the above two categories.
Equalizing of the output spectrum can be done in several ways, such as through the use of optical tunable filters. Another way to equalize the output spectrum is through the use of a Mach-Zehnder Interferometer (MZI), as described in U.S. Pat. No. 6,384,964 issued to Mizuno, et al on 7 May 2002.
When SLEDs and ELEDs are used in a broadband light source (as in the case of the second category of broadband light sources described above), the need for spectrum equalization becomes a cause of concern as the SLEDs and the ELEDs normally have a Gaussian-like profile.
Although a flat spectrum can be achieved by combining several SLEDs with different attenuation ratios as described in U.S. Pat. No. 6,570,659, the central wavelengths of the SLEDs used have to be close enough to each other to output a flat spectrum. More SLEDs will have to be used if a flatter and wider spectrum is required, resulting in an increase in the complexity of the diode driver circuitry required. This increases the cost of the broadband light sources.
Further an increase in the number of SLEDs also gives rise to higher insertion loss at the attenuator, thus diminishing the advantage of having a high output power when more SLEDs are used.
Most available spectrum equalizers have a specific effective spectrum equalizing range that corresponds to the spectrum of an optical amplifier, such as the EDFA, used in the broadband light source. These spectrum equalizers will have a different spectrum range from that of the SLEDS, and more importantly, they are not designed with a minimal attenuation. Thus, such equalizers will not prove useful to equalize the Gaussian-like spectrum of the SLED with a pre-specified spectral flatness and high output power.
There is thus a need to have an equalizer that can yield a flat output spectrum over a wide bandwidth, while a small number of SLEDs are used.